1. Field of the Invention
The present invention relates to an optical-disc apparatus for recording information in and reproducing information from an optical disc, wherein rotational speed of the optical disc is appropriately controlled.
2. Description of Related Art
Recently, the optical discs used as a information recording medium include not only a CD (Compact Disc) and a LD (Laser Disc) but also an MD (Mini Disc). The MD is a recordable and reproduceable magneto-optical disc, which has been developed based on advances in optical modulation technology and magnetic-field modulation technology. The MD has recently come into wide use. The MD is used as a commercial product in public-use as a rewriteable music recording medium.
A system for driving the MD is basically similar to an optical-disc apparatus (magneto-optical-disc apparatus). However, in order to record an amount of information corresponding to 74 minutes on the MD having a diameter of 64 mm, the system additionally performs voice-information compression and includes a memory for preventing problematical skipping of voice information which may occur due to vibration applied to the system.
FIG. 1 shows a block diagram of an optical disc apparatus in the related art. In the optical disc apparatus 11 shown in FIG. 1, laser beams emitted by a pickup 14 are incident on an optical disc 13, which is rotated by a disc motor 12 at a controlled rotational speed. Reflected light is converted into a reproduced signal by the pickup 14. The signal is supplied to a synchronization detecting circuit 16, a PLL (Phase Locked Loop) circuit 17 and a disc servo circuit 19, via an analog waveform-shaping circuit 15.
The PLL circuit 17 compares the reproduced signal supplied by the analog waveform shaping circuit 15 with a reference signal which is generated by an internal oscillator (VCO: Voltage Controlled Oscillator). Thus, a reproduced clock signal for demodulation is generated as a result of causing the reference signal to be in synchronization with the reproduced signal, and is supplied to the synchronization detecting circuit 16, a signal processing circuit 18 and the disc servo circuit 19. The synchronization detecting circuit 16 uses the reproduced clock signal from the PLL circuit 17 so as to process the reproduced signal, and then produces a main signal which is thus in synchronization for each bit with the reproduced clock signal and is transmitted to the signal processing circuit 18.
The signal processing circuit 18 performs the processes of error correction, D-A conversion and so forth on the thus-supplied main signal. During these processes, address information is extracted from the main signal, which information is then supplied to a CPU (Central Processing Unit) 21 via a track-address detecting circuit 20. The signal processing circuit 18 includes a memory for preventing problematical skipping of voice information. This memory stores voice information for several seconds successively and uses the thus-stored information to fill in a blank section of voice information which may occur due to jumping of the pickup due to vibration or the like. Thus, skipping resulting in a lack of voice information can be prevented.
A disc servo circuit 19 produces a servo signal as a result of comparing the reproduced clock signal from the PLL circuit 17 with a reference clock signal generated by a system clock 25, and supplies it to the disc motor for controlling the rotation speed of the optical disc 13 at a Constant Linear Velocity (CLV) such that the linear velocity of a recording/reproducing track of the disc 13 passing in front of the pickup is fixed over all radiuses of the disc 13.
In the MD used only for data reproduction, similar to the CD, all areas have a pit structure. In the MD for both data recording and data reproduction, a read-in area has a pit structure, but each recording area and read-out area has a groove structure. If the optical disc 13 is a rewriteable MD, a pit tracking mode and a groove tracking mode are used for the pit structure and groove structure, respectively. When a tracking operation is performed on the pit structure, a protruding portion on the disc be traced. In contrast to this, when the tracking operation is performed on the groove structure, a recessed portion on the disc is traced. Accordingly, polarity of an error signal used in the tracking operation in the pit tracking mode is inverse of the polarity of the error signal used in the tracking operation in the groove tracking mode. (Such a polarity of the error signal as used in the tracking operation will be referred to as a servo polarity, hereinafter.)
Further, the disc servo circuit 19 performs a servo control for an EFM (Eight-to-Fourteen Modulation) signal when the pit structure is scanned, but performs the servo control for an ADIP (ADdress In Pregroove) signal when the groove structure is scanned. For performing such changing of control, the disc servo circuit 19 produces the servo signal for the disc-rotation-speed control.
The CPU 21 controls a pickup servo circuit 22 which thus uses the reproduced signal from the pickup 14 and thus performs a focus control operation, a tracking control operation and a pickup forwarding/reversing control operation. Further, the pickup servo circuit 22 performs the tracking control operation, where the servo polarity of the tracking error signal is changed as the structure being scanned is changed between the pit and groove structures.
When recording, the signal processing circuit 18 performs the processes of A-D conversion, data (voice information) compression, EFM and so forth and thus supplies a main signal to a head driving circuit 23. A magnetic-modulation recording method is used when recording information in the MD. Accordingly, the recording operation is performed through a head 24 which is located above an upper surface of the optical disc 13. The head driving circuit 23 controls the head 24 for this purpose.
The disc-rotation-speed control according to the CLV is simply the servo control for the EFM signal for the MD for only reproduction because in such type of MD, all of the areas have the pit structure. However, for the MD capable of recording and reproduction, it may be necessary to change the servo control between that for the EFM signal and that for the ADIP signal in response to a change of an area being scanned by the pickup 14.
The pickup will trace a protruded portion on the optical disc 13 in an area of the pit structure, and trace a recessed portion on the disc in an area of the groove structure. The change is performed by appropriately changing the servo polarity of the error signal. If this change is not performed appropriately and thus the servo polarity of the error signal is inverted, a portion of the disc at which no significant signal is recorded is traced by the pickup 14. As a result, no significant signal is obtained.
If the pickup 14 unexpectedly moves to an area of the groove structure, during scanning of an area of the pit structure, due to some cause, the servo polarity of the tracking error signal may not be appropriately changed and also the disc-rotation-speed control may not be appropriately changed from that for the EFM signal to that for the ADIP signal. If the change is not appropriately performed as mentioned above, no significant signal is obtained above and thus disc-rotation-speed control cannot be appropriately performed. In other words, the disc rotation speed is out of control.
As a result, the disc motor 12 drives the optical disc 13 at a high speed. Once the apparatus enters such a situation, if the above-mentioned change is correctly performed, due to respondable frequency bands of the focus servo control operation and tracking control operation and so forth, it is not easy to return the apparatus to a normal state and thus realize a CLV-servo locked state which will be described later.